1. Technical Field of the Invention
The present invention relates to a fast battery charger with temperature detection and, more particularly, relates to a fast battery charger that senses the second derivative of temperature of the charged battery.
2. Description of the Related Art
Rechargeable batteries can be charged by two types of battery chargers, trickle chargers and fast chargers. A trickle charger slowly charges a battery with a current on the order of low hundreds of milliamperes (mA). A trickle charger takes over half a day or so to charge a typical rechargeable radio battery. A fast charger charges a battery with about an Ampere or so of current and takes about one hour to charge a typical rechargeable radio battery. Because fast chargers charge a battery about a magnitude faster than trickle chargers, fast chargers are preferred. However, fast chargers pose additional problems. A battery can be overcharged by a fast charger because of the high current used for charging. Because the trickle charger has a low current, the battery can itself prevent further charging if the cells are designed to accept a continuous overcharge. A fast charger, on the contrary, requires circuitry to discontinue charging of the battery. Without such circuitry, the fast charger will overcharge the battery, risking damage to the battery. Overcharging can shorten battery life by heating up and deteriorating a separator in the cells or by causing high pressure to vent the cell resulting in non-recoverable lost capacity.
Early techniques for controlling the charge of a battery used timers. A timer limited the charge of the battery to a fixed period of time such as, for example, an hour. After an hour, the timer would stop the charge amperage. These timer circuits, however, assumed they were starting with an empty battery. These timer circuits overcharged a full or partly charged battery.
U.S. Pat. No. 4,806,840 discloses circuits for controlling charge based on a first derivative of the voltage of the battery. The first derivative sensed the rate of change of the voltage. When a predetermined negative rate of change of the voltage was sensed, the charging is discontinued. However, the battery must be overcharged before the charging is discontinued. To reduce the overcharge, U.S. Pat. No. 4,503,378 senses the second derivative of voltage to control the charge. The second derivative detects where the slope of the battery voltage changes from increasing to decreasing. Thus charging is discontinued when only the second derivative of voltage switches. This technique of sensing voltage, however, does not charge the battery to its fullest capacity and is susceptible to premature tripping.
Other circuits detect the temperature of the battery and ceased charge when the battery reached a fixed temperature such as, for example, 40.degree. C. These charging circuits also cause the battery to sometimes be undercharged or overcharged. A charge circuit for controlling the charge based on a first derivative of sensed temperature of the battery is disclosed by U.S. Pat. Nos. 3,852,652 and 4,755,735. A first derivative of temperature provides a signal representative of the rate of change of the temperature of the battery. When the rate of the change of the temperature exceeds a predetermined threshold, charging of the battery is discontinued.
When the battery temperature is significantly below ambient temperature, however, the previously known techniques of charge control prematurely trip and prevent full charge of the battery. When the battery is brought indoors for charging after being outdoors in a cold automobile, battery temperature will rapidly rise to indoor ambient temperature. A rapid rate of battery temperature change causes a high first derivative, for example, that reaches the predetermined threshold early and prematurely trips with an insufficient charge on the battery.